Method of communicating between secured computer systems, a computer network infrastructure and a computer program product

A method communicates between secured computer systems in a computer network infrastructure. Data packets are transmitted between a plurality from a group of processing computer systems, wherein such a transmission is performed by at least one broker computer system. The data packets are advantageously routed via at least one relay system connected upstream or downstream of the broker computer system in a transmission path of the data packets. All from the group of processing computer systems keep predetermined network ports at least temporarily closed so that access to a respective processing computer system via a network by the network ports is prevented. The relay system keeps predetermined network ports closed at least to the broker computer system, which has the relay system connected downstream so that access to the relay system via a network by the network ports is prevented.